Java Language Collections Constructing collections from existing data


Example

Standard Collections

Java Collections framework

A simple way to construct a List from individual data values is to use java.utils.Arrays method Arrays.asList:

List<String> data = Arrays.asList("ab", "bc", "cd", "ab", "bc", "cd");

All standard collection implementations provide constructors that take another collection as an argument adding all elements to the new collection at the time of construction:

List<String> list = new ArrayList<>(data); // will add data as is
Set<String> set1 = new HashSet<>(data); // will add data keeping only unique values
SortedSet<String> set2 = new TreeSet<>(data); // will add data keeping unique values and sorting
Set<String> set3 = new LinkedHashSet<>(data); // will add data keeping only unique values and preserving the original order

Google Guava Collections framework

Another great framework is Google Guava that is amazing utility class (providing convenience static methods) for construction of different types of standard collections Lists and Sets:

 import com.google.common.collect.Lists;
 import com.google.common.collect.Sets;
 ...
 List<String> list1 = Lists.newArrayList("ab", "bc", "cd");
 List<String> list2 = Lists.newArrayList(data);
 Set<String> set4 = Sets.newHashSet(data);
 SortedSet<String> set5 = Sets.newTreeSet("bc", "cd", "ab", "bc", "cd");

Mapping Collections

Java Collections framework

Similarly for maps, given a Map<String, Object> map a new map can be constructed with all elements as follows:

Map<String, Object> map1 = new HashMap<>(map);
SortedMap<String, Object> map2 = new TreeMap<>(map);

Apache Commons Collections framework

Using Apache Commons you can create Map using array in ArrayUtils.toMap as well as MapUtils.toMap:

 import org.apache.commons.lang3.ArrayUtils;
 ...
 // Taken from org.apache.commons.lang.ArrayUtils#toMap JavaDoc

 // Create a Map mapping colors.
 Map colorMap = MapUtils.toMap(new String[][] {{
     {"RED", "#FF0000"},
     {"GREEN", "#00FF00"},
     {"BLUE", "#0000FF"}});

Each element of the array must be either a Map.Entry or an Array, containing at least two elements, where the first element is used as key and the second as value.

Google Guava Collections framework

Utility class from Google Guava framework is named Maps:

 import com.google.common.collect.Maps;
 ...
 void howToCreateMapsMethod(Function<? super K,V> valueFunction,
           Iterable<K> keys1, 
           Set<K> keys2, 
           SortedSet<K> keys3) {
     ImmutableMap<K, V> map1 = toMap(keys1, valueFunction); // Immutable copy
     Map<K, V> map2 = asMap(keys2, valueFunction); // Live Map view
     SortedMap<K, V> map3 = toMap(keys3, valueFunction); // Live Map view
 }
Java SE 8

Using Stream,

Stream.of("xyz", "abc").collect(Collectors.toList());

or

Arrays.stream("xyz", "abc").collect(Collectors.toList());